Dobbies of the foregoin are disclosed in U.S. Pat. Nos. 3,724,511 and 3,730,231.
The radial engaging and disengaging movement of the coupling part in the eccentric ring, during a standstill of the drive shaft, is effected according to a pattern by a switching member which has a circular opening arranged approximately concentrically to the drive shaft. The circular opening has over its entire periphery a collar which projects beyond the rotary switching member and constantly engages a sliding groove in the coupling part. The switching member is thereby supported tiltably in the plane of its circular opening, and its movement during the control operation of the engaging and disengaging of the coupling part is automatically effected by the control system of the dobby during the standstill of the drive shaft. During rotation of the drive shaft, the switching member is freely movable, and is moved by the coupling part in correspondence with the movement of the coupling part as the coupling part pivots 180.degree. with the drive shaft due to the cooperating sliding groove and the collar, which assures that the switching member can properly effect its control function at any time with respect to the coupling part. In this embodiment, the coupling part lies, during its 180.degree. swinging movement, in a groove in the eccentric ring and is taken along by the drive shaft. To avoid disengagement of the coupling part from the groove of the drive shaft, springs are arranged in the coupling part. In addition, the connecting rod has resilient, semi-circular guide rails, against which the coupling part can rest.
In German Pat. No. 30 01 310, the coupling part is held by a pawl in its swinging movement.
German Offenlegungsschrift No. 2 256 863 (corresponds to U.S. Pat. No. 3,804,128) and German Auslegeschrift No. 2 841 278 both suggest replacing the aforementioned switching member having a circular opening with two rotary switching arms, at the ends of which are arranged groove stones which, during the duration of the control operation of the coupling part, engage grooves of the coupling part. During the rotary movement of the coupling part, these rotary switch arms are not operatively connected to the coupling part. This fact and the repeated moving of the groove stone into the groove of the coupling part at the beginning of each control operation presents an instability factor, which requires additional control and safety members. In particular, in the case of high speeds, an orderly control is no longer assured. In addition, it is difficult, due to the prescribed pitch, namely, the limited heddle-frame thickness in a weaving machine, to properly mount these additional control and safety members.